Simple mobile radio devices (cell-phones) serve only one standard (one frequency band of a mobile radio system) and they are therefore only unrestrictedly operable in regions wherein sufficient net coverage for this standard is available. Multi-band mobile radio devices covering several frequency bands (of a mobile radio system) are suitable, for purposes of better accessibility, in regions with incomplete net coverage and/or for increasing the capacity in areas with many users. So-called dual band and triple band cell-phones function by means of the same transmission process (for example GSM), but they can transmit and receive in different frequency bands and are therefore equipped for several standards, for example simultaneously for GSM1800 (DCS, Digital Cellular System, 1800 MHz) and GSM900 (EGSM, 900 MHz) or even additionally for GSM1900 (PCS, Personal Communication System, 1900 MHz) and/or GSM850 (850 MHz).
At present, communication devices with a multi-mode transmission system are also being developed, which devices are suitable for operating in several mobile radio systems of the same generation or of different generations (e.g., GSM combined with UMTS), in that a switch is provided on the input side and/or the antenna side, which switch alternatively connects an antenna with the signal paths associated with different mobile radio systems. In heretofore known communication devices with a multi-mode transmission system, which are also applicable for operation via the UMTS transmission process, which implies continuous wave signal transmission, the UMTS components, in particular band pass filters for 2000 MHz, are usually designed on the basis of ceramic microwave elements. These are connected behind a front-end circuit, so that the corresponding interface constitutes a potential source for impedance matching problems and therefore also for signal losses, since, e.g., the length of the signal paths are not fixed.
A front-end circuit is understood to be the antenna-side part of a communication device, which connects the shared antenna with the filters and the latter with the possibly different signal processing paths for different operating modes and access processes, in particular the LNA (low noise amplifier) for the reception path or the PA (power amplifier) for the transmission path, and which furthermore includes the switches necessary for switching between the access and the operating processes.
Numerous existing wireless transmission systems, in particular mobile radio systems, can differ both in terms of the transmission standard and in terms of the frequency bands being employed (multi-mode/multi-band systems). This makes use of various access methods (multiplexing methods) for the transmission of different data in a communication channel, for example CDMA (Code Division Multiple Access), WCDMA (Wideband CDMA), TDMA (Time Division Multiple Access) or FDMA (Frequency Division Multiple Access). Combinations of access methods, e.g. TD-CDMA (Time Division Code Division Multiple Access) in the transmission of UMTS data, are also known.
These different access methods can additionally comprise different duplexing methods, in order to separate the transmitted and the received data and to enable a simultaneous transmit and receive operation in the communication device. Known duplexing processes are FDD (Frequency Division Duplexing) and TDD (Time Division Duplexing). Some standards also use a duplexing processes with a mixed FDD/TDD operation, in which different frequency bands are provided for the transmit and receive operations, but in which the transmitted and received signals are additionally temporally separated from each other and are transmitted or received in so-called different time slots.
In the case of known multi-band mobile radio device standards with a mixed FDD/TDD duplexing operation, access to a shared antenna for transmitting (TX) and receiving (RX) is usually achieved via an HF switch. A transmission system then uses a pair of (frequency) bands, in which the frequencies are designated for transmitting and receiving. If the band pair of a system is sufficiently far away from other bands (other systems) (typically about 1 octave), the filters and the signal processing paths for this band pair can be interconnected in an impedance neutral manner (e.g., through a diplexer) which is separate from the remaining ones and they can be connected with the shared antenna. The use of a diplexer is generally applicable for selecting the frequency band and/or pre-selecting different systems whenever the frequency interval between the frequency bands amounts to about 1 octave. A frequency interval of 1 octave means a doubling of the frequency. For example, a system in the 1 GHz band and a system in the 2 GHz band are separated from each other by 1 octave. However, the 1 GHz range is understood to include all frequency bands that are between 800 and 1000 MHz, while a 2 GHz system comprises all bands between 1700 and 2200 MHz.
The band pairs of other standards, which lie closer to a first band pair, are usually separated from each other in known multi-band devices by means of an additional diplexer circuit and from the rest of the front-end circuit by means of an active switch connected in front of the diplexer circuit, in order to protect the respective receiver from the transmitted signal strength in the first-mentioned signal path, in particular with overlap of the transmission range of a band pair with the reception range of another band pair.
The increase in the number of frequency bands to be included in a mobile radio usually requires the development of a new chip set. The chip set can consist of one or more HF ICs and is suitable for signal processing (e.g. transmitted signal production, modulation/demodulation, mixing and amplification/power amplification) of the appropriate number of frequency bands.